cut: add BENCHMARKING.md

and minor refactoring

src/uu/cut/BENCHMARKING.md

@@ -0,0 +1,46 @@
+## Benchmarking cut
+
+### Performance profile
+
+In normal use cases a significant amount of the total execution time of `cut`
+is spent performing I/O. When invoked with the `-f` option (cut fields) some
+CPU time is spent on detecting fields (in `Searcher::next`). Other than that
+some small amount of CPU time is spent on breaking the input stream into lines.
+
+
+### How to
+
+When fixing bugs or adding features you might want to compare
+performance before and after your code changes.
+
+-   `hyperfine` can be used to accurately measure and compare the total
+    execution time of one or more commands.
+
+    ```
+    $ cargo build --release --package uu_cut
+
+    $ hyperfine -w3 "./target/release/cut -f2-4,8 -d' ' input.txt" "cut -f2-4,8 -d' ' input.txt"
+    ```
+    You can put those two commands in a shell script to be sure that you don't
+    forget to build after making any changes.
+
+When optimizing or fixing performance regressions seeing the number of times a
+function is called, and the amount of time it takes can be useful.
+
+-   `cargo flamegraph` generates flame graphs from function level metrics it records using `perf` or `dtrace`
+
+    ```
+    $ cargo flamegraph --bin cut --package uu_cut -- -f1,3-4 input.txt > /dev/null
+    ```
+
+
+### What to benchmark
+
+There are four different performance paths in `cut` to benchmark.
+
+-   Byte ranges `-c`/`--characters` or `-b`/`--bytes` e.g. `cut -c 2,4,6-`
+-   Byte ranges with output delimiters e.g. `cut -c 4- --output-delimiter=/`
+-   Fields e.g. `cut -f -4`
+-   Fields with output delimiters e.g. `cut -f 7-10 --output-delimiter=:`
+
+Choose a test input file with large number of lines so that program startup time does not significantly affect the benchmark.

src/uu/cut/src/cut.rs

@@ -205,7 +205,7 @@ fn cut_fields_delimiter<R: Read>(
             return Ok(true);
         }
 
-        for &Range { low, high } in ranges.iter() {
+        for &Range { low, high } in ranges {
             if low - fields_pos > 0 {
                 low_idx = match delim_search.nth(low - fields_pos - 1) {
                     Some(index) => index + input_delim_len,

src/uu/cut/src/searcher.rs

@@ -33,9 +33,9 @@ impl<'a> Iterator for Searcher<'a> {
                 if self.needle.len() == 1
                     || self.haystack[match_idx + 1..].starts_with(&self.needle[1..])
                 {
+                    let match_pos = self.position + match_idx;
                     let skip = match_idx + self.needle.len();
                     self.haystack = &self.haystack[skip..];
-                    let match_pos = self.position + match_idx;
                     self.position += skip;
                     return Some(match_pos);
                 } else {